Rome Total War: Barbarian Invasion (often abbreviated BI) is the first official expansion pack released by the Creative Assembly for the game Rome: Total War.
Barbarian Invasion was released on September 30, 2005.

Contents

It is the year 363 AD. After decades of internal strife, the Roman Empire has been split in two. In Europe, Barbarian tribes are on the move, ready and willing to cross the Rhine and Danube and attack the divided Romans. Huge armies of Barbarian Horsemen threaten the once great Empire, as rivalry between the two weakens their borders.

Christianity has become the state religion of the Roman empire. Paganism is still highly prevalent within the Barbarians and much of the Empire, especially in frontier towns and forts. Religion will be a major issue, especially with the resurgent Sassanid Empire's worship of Zoroaster and their persecution of Christians, and the different denominations of Christianity amongst the barbarians.

Night battles. Generals can now pick up traits that allow them to fight at night, thereby granting allied armies bonus and conferring potential disadvantages on the enemy.

Emerging Factions. Some factions can emerge in the game as the result of revolts or historical events. For example, the Ostrogoths, Western Roman Rebels and Eastern Roman Rebels will emerge if a settlement owned by the Goths, WRE or ERE rebels. The Slavs appear in-game in 410 AD, and the Romano-British emerge when all Roman-owned settlements in Britain are conquered by another faction.

New factions, Generals, and Units. By 363 AD Europe, North Africa, and Middle East had drastically changed. New "barbarian" tribes were migrating into Europe, the Sassanids had risen in Persia, and the Roman Empire had been split in two, and had even come to blows. All this is reflected in the hundreds of new units available. General's units are now also recruitable.

Loyalty attribute. This is added to all Roman Family Members, and measures their chance to rebel or be bribed.

Swimming. Many units can now swim, and can cross rivers without using a bridge or ford. This alters bridge battles significantly. An exhausted unit may drown when attempting to swim.

Recruitable Generals. All factions can now recruit generals, providing they have the required level of Stable. They essentially act as a Captain that can gain character traits and ancillaries.

New traits and ancillaries. Three and a half centuries onwards from the end of Rome:Total War, family members now gain new traits (though old ones can still be gained). The most important is the Night Fighter trait, allowing a General to fight at night.

Hordes. Many Barbarian factions now have the ability to "horde". A larger overview of Hording can be found here.

Religion. Religion is now a far more important aspect than in Rome: Total War. For more information, go here.

The factions in Barbarian Invasion are completely new, for the most part. There is still a Persian Empire, but in a different dynasty. The Romans still exist, but instead of three families and a Senate, there are the Eastern and Western parts of the Roman Empire. These two factions are completely separate, and will go to war against each other.